Several physical methods are known for the determination of the specific gravity of urine, for example the determination with hydrometers, urinometers, pyknometers and refractometers. These methods are admittedly of sufficient exactitude but require a high instrument expense and are time-consuming and costly due to the necessary cleaning and calibration of the apparatus.
The measurement of the specific gravity with the aid of so-called test strips, which can also be evaluated visually, is simpler. The test strips have a test area which contains appropriate reagents. The determination can take place in a simple way by dipping the test strip into a urine sample and observing the coloration of the test area. Furthermore, the test strips offer the advantage of being combinable with other urine tests, for example for the detection of glucose, leukocytes, blood and the like, in a multiple test strip with a corresponding number of test areas spaced from one another. The measurement of the specific gravity of urine can then take place, with a saving of time, in one step, in addition to the measurement of these other parameters. The evaluation can also be carried out with the use of appropriate apparatus.
A method for determining the specific gravity of urine with a test strip is described in U.S. Pat. No. 4,015,462. A carrier matrix is provided with osmotically breakable microcapsules which contain a liquid; when such a matrix is brought into contact with a sample solution of lower osmolality, the hydrostatic pressure increases in the microcapsules, the walls of which consist of semi-permeable membrane material. This leads to a rupture of the microcapsules. The rupture of the microcapsules and the resulting release of a dye dissolved in the liquid leads to a coloration of the matrix. The intensity of the coloration is proportional to the osmolality or to the specific gravity of the sample solution. A disadvantage of this method is the use of a carrier matrix provided with microcapsules since the production thereof is very difficult because of the necessary exactitude.
A further method for determining the specific gravity or the ionic strength with a test strip is described in U.S. Pat. No. 4,318,709 and the corresponding Federal Republic of Germany Patent Specification No. 29 44 980. In this method the sample is mixed with a reagent which contains a weakly acidic or weakly basic polyelectrolyte polymer neutralized to an extent of at least 50% and an indicator. The fundamental principle is the proportionality between the specific gravity and the ionic strength of an aqueous solution.
Polyelectrolyte polymers are there designated as being polymers with ionic groups, polyacrylic acid and polyvinylamine being mentioned by way of example.
European Patent Specification No. 0,114,315 describes a reagent which contains a weakly basic polyelectrolyte polymer neutralized by a strong organic acid and an indicator.
European Patent Specification No. 0,114,316 discloses a reagent which contains a weakly acidic polyelectrolyte polymer, in which at least one carboxyl group is present in the form of an ammonium salt, and an indicator.
European Patent Specification No. 0,023,631 discloses a reagent for determining the ionic strength or the specific gravity. Besides a strongly acidic or strongly basic polyelectrolyte polymer, it contains a buffer substance which ensures a pH value of at least 6.5, as well as a pH indicator.
However, methods which make use of a reagent which contains a polyelectrolyte polymer have the disadvantage that the color brought about by the pH shift changes in the course of time. The result of this color change is that the reading for the specific gravity after 1 minute differs very greatly from the reading after 2 or even 5 minutes.
Thus, if the test is carried out by persons who are not specifically trained for the purpose, such as patients, and the evaluation is not always undertaken at the same time as indicated by the producer of the strip, the readings can lead to seriously erroneous interpretations of the state of the health of a patient.
Therefore, there is a need for a method for determining the ionic strength or the specific gravity of aqueous liquids which provides test results which remain constant during the test over a comparatively long period of time.